Maxwell forces in the electrical machines and in particular in electrical machines air gaps generate high frequency (up to 6000 Hz) vibrations of the outer structure, e.g. of the rotor or stator depending on the topology of the electrical machine. This can result in strong vibration and noise levels.
An article of J. Le Besnerais, V. Lanfranchi, M. Hecquet, P. Brochet, “Characterization and reduction of audible magnetic noise due to PWM supply in induction machines” in IEEE Trans. on Ind. Elec, n 99, August 2009 refers to audible noise arising mainly from air-gap radial Maxwell forces that excite the stator in the audible range (magnetic noise). Analysis shows that by proper choice of a PWM switching frequency, a resonance of PWM exciting forces and corresponding stator modes can be avoided and hence the magnetic noise be reduced. However, such a noise reduction deals with resonance problems, but not with forced vibrations.
In view of the above-described situation, there exists a need for an improved technique that enables to provide an efficient electrical machine while substantially avoiding or at least reducing one or more of the above-identified problems.